Bacteria have evolved complex strategies to compete and communicate with one another. One important mechanism of inter-bacterial competition is mediated by contact-dependent growth inhibition (CDI) systems. CDI systems are found in a wide variety of Gram-negative bacteria, including many important human pathogens. CDI is mediated by the CdiB/CdiA family of two-partner secretion proteins. CdiB is an Omp85 outer-membrane protein that is required for the export and assembly of the CdiA exoprotein onto the cell surface. CdiA binds to receptors on susceptible bacteria and then delivers its C-terminal toxin domain (CdiA- CT) into the target cell. CDI systems also encode CdiI immunity proteins, which specifically bind to the CdiA- CT and neutralize toxin activity, thereby protecting CDI+ cells from auto-inhibition. Remarkably, CdiA-CT sequences are highly variable between bacteria, as are the corresponding CdiI immunity proteins. Current analysis indicates that CDI systems encode at least 120 distinct CDI toxin-immunity families. This application proposes a combination of genetic, biochemical and ultrastructural analyses to gain mechanistic insights into cell-cell interactions and CdiA-CT toxin delivery during CDI. This research will significantly increase our understanding of the ecology and evolution of bacterial pathogens and could inform novel approaches to antimicrobial therapy.